This invention relates to a magnet apparatus for producing a magnetic field for use in magnetic resonance imaging (MRI). The apparatus is of the type having two oppositely arranged permanent magnets.
MRI is a process to form sectional images of biological tissues placed in a uniform magnetic field. In the current medical treatments this process is very useful as a diagnostic tool to obtain cross-sectional images of selected parts of the human body.
A MRI apparatus has a magnet unit to produce a uniform magnetic field in a limited but sufficiently wide space within the apparatus, and for this purpose several types of magnet units are known.
Some MRI apparatus employ a pair of permanent magnet discs each of which is magnetized in the direction of thickness. In a yoke structure the two magnet discs are arranged parallel and opposite to each other, usually in a vertical direction, such that a pole face of one magnet disc faces the opposite pole face of the other. For example, the yoke structure has top and bottom plate parts to which the two magnet discs are fixed, respectively, and a plurality of pillar parts which connect the top and bottom plate parts. To produce a uniform magnetic field within a cylindrical space between the two magnet discs, a pole plate (pole piece) in the form of a disc having an annular protuberance in a peripheral region is placed on the pole face of each magnet disc. The pole plate is made of a ferromagnetic material such as soft iron. In some cases each of the permanent magnet discs has a hole in a central region, and for each magnet disc the yoke structure has an aperture which is in alignment with the hole in the magnet disc. For diagnosis with MRI apparatus of this type the human body is partly and horizontally inserted into the space between the two magnet discs through openings in the yoke structure.
In operating a MRI apparatus of the above described type it is not easy for a MRI engineer to observe the state of the object of imaging. For instance, when the object of imaging is the head part of a human body the engineer is obliged to look into the imaging space in the apparatus in a direction along the lying patient, but it is difficult to clearly grasp the state of the head part or the patient's expression since the spacing between the two magnet discs in the apparatus is only about 50 cm. If it is intended to resolve this inconvenience by widening the spacing between the two magnet discs there arises a problem that the magnetic field in the apparatus is degraded in both intensity and uniformity. If the problem should be solved by using large-sized permanent magnet discs there arises another problem that the MRI apparatus becomes high in production cost and undesirably large in size.